(1) Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and, more particularly, to a method of fabricating a silicon (Si) bipolar transistor.
(2) Description of the Related Art
A previously known technique of enhancing the current gain factor h.sub.FE of a bipolar transistor is to form a thin silicon oxide film on a surface of the base region and thereafter to form an emitter region by way of impurity diffusion through the oxide film. More specifically, an opening is made in the insulating layer covering the base region so as to expose a part of the base region, and then the silicon oxide film is formed on the exposed surface. A polycrystalline silicon (also called "polysilicon") layer containing impurities is formed on the silicon oxide film, and the impurities contained in the polysilicon are diffused into the base region through the silicon oxide film to form the emitter region.
In such a transistor, the thickness of the silicon oxide film to be formed on a single-crystal layer and its characteristics play an important role to improve such factor as h.sub.FE of the transistor.
Japanese Patent Application Kokai No. Hei 2 (1990)-210,820 proposes two methods of forming such a silicon oxide film.
One method is as explained hereinafter. First, an n-or p-type silicon substrate (100) having a diameter of 4 inches and a resistivity of 0.01.about.0.02.OMEGA..multidot.cm is RCA-cleansed and thereafter is introduced in an MBE (Molecular Beam Epitaxy) growth equipment which is provided with a 40 cc electron gun type silicon evaporator. An amorphous silicon is deposited 10 .ANG. in thickness on the silicon substrate, and thereafter the natural oxide film on the surface is removed by the cleaning treatment under the condition of 800.degree. C. and one minute. An epitaxial layer to be used as a buffer layer is grown 3000 .ANG. in thickness under the condition of 500.degree. C. While the substrate temperature is lowered to room temperature and oxygen gas having a purity of 99.9999% is allowed to leak from a nozzle into the MBE growth equipment at the partial pressure of oxygen of 5.times.10.sup.-5 Torr, a silicon molecular beam is projected from the electron gun type silicon evaporator to the buffer layer to form a silicon oxide film. The oxygen leaked in the growing chamber is exhausted. The silicon molecular beam is further projected in an atmosphere of the vacuum degree of 1.times.10.sup.-9 Torr and the substrate temperature of 500.degree. C. to deposit a polysilicon.
The other method disclosed is a low pressure oxidation technique in which only oxygen molecules are projected to a heated silicon substrate. In this method, a cleaned silicon (100) plane is oxidized for about 30 minutes under the condition of the substrate temperature of 500.degree. C. and the oxygen partial pressure of 5.times.10.sup.-5 Torr. Thereafter, the silicon molecular beam is projected as in the first method to deposit the polysilicon.
The above conventional methods of forming the interfacial silicon oxide film suffer from the following problems.
The interfacial silicon oxide film formed by the first method of the prior art contains more silicon than the silicon oxide film having an accurate stoichiometric composition ratio, i.e., SiO.sub.2, as disclosed in FIG. 1(a) of Japanese Journal of Applied Physics Vol. 28, No. 10, October 1989, pp. L1678-L1681, "Si/SiOx/Si Hole-Barrier Fabrication for Bipolar Transistors Using Molecular Beam Deposition". This state is shown in FIG. 12. In FIG. 12, a curve 120 is directed to the case where the composition ratio of silicon (Si) to oxygen (O) is 1:2. The remaining curves are directed to the cases with the deposition temperature varied according to the conventional method. As seen, lower the deposition temperature is, more the composition ratio of the film approaches the SiO.sub.2. However, even if the silicon oxide film is formed at room temperature (RT), its composition is silicon-rich since the bonding energy has a peak in the neighborhood of 100 eV. As described later, the silicon bipolar transistor having the silicon-rich silicon oxide film at the boundary between the polysilicon and the single-crystal silicon has a problem that its current gain factor h.sub.FE cannot be improved.
On the other hand, although the deposition temperature in the second method of the prior art is not so high, oxygen molecules are projected for 30 minutes at 500.degree. C., so that its processing capability is very low.